1. (Field of the Invention)
The present invention relates to silicon balls used as substrates for semiconductor circuits and solar cells and a method of making such silicon balls.
2. (Description of the Prior Art)
As a semiconductor technology for the next generation, globular silicon semiconductors have recently come to be spotlighted. Globularizing of a conventional plane silicon wafer has an advantage in that the manufacturing cost that is brought about infinite plant and equipment investment can be reduced and that utilization of silicon material can be maximized as a result of the use of a relatively large spherical surface having a large specific surface area.
In order to use globular silicon as a semiconductor circuit substrate, a surface property comparable to the silicon wafer and a high shape precision are required.
However, the globular silicon satisfying such requirements and a practical method of making such globular silicon have not yet been made available.
Silicon balls of single crystal silicon are generally manufactured by a lapping process of lapping workpieces of single crystal silicon to produce lapped balls and a subsequent finishing process for finishing the lapped balls, that is, a polishing process of polishing the lapped balls to produce the polished balls.
A first problem inherent in the lapping process and associated with the lapped balls will be discussed.
To increase the shape precision of a globular body, a grinding is generally performed. However, processing of silicon balls with the use of a generally utilized metal grinder for cast iron or carbon steel, no surface roughness will be improved even though very finely divided abrasive particles are used. The reason therefor appears to result from that in the practice of the metal grinding process, silicon balls tend to receive a relatively large processing force from the grinder and the abrasive particles and that the silicon balls tend to be agglutinated.
To alleviate the foregoing problems, the inventors of the present invention have suggested a lapping process using an alkaline solution and a lapping process using a resinous material for a material of the surface plate. However, even with these suggested methods, a sufficient shape precision and a sufficient surface property have not yet been attained.
More specifically, although the processing with the use of the alkaline solution utilizes a dissolution, a chemical reaction takes place uniformly on the workpieces and; therefore, compensation for shape is limited. Also, the surface roughness to be controlled by the alkaline concentration is limited and, while the solution concentration has to be lowered to secure a minute surface roughness, virtually no processing proceeds if the concentration is lowered to a certain extent.
In the case of the use of the resinous surface plates, since the processing force which the silicone balls receive from the surface plate and/or the abrasive particles embedded therein is lessened as a result of elastic deformation of the resin, and since no agglutination occur between the workpieces and the surface plate without strongly impairing the surface of the balls, improvement in surface roughness can be expected. However, the extent to which the shape precision can be increased is limited because of the elastic deformation of the resin. Also, the use of very finely divided grinding particles for the purpose of securing a minute surface roughness will result in malfunction of the grinding particles during the processing and, as a result thereof, virtually no processing proceeds.
Also, in order to secure the surface property comparable to the silicon wafer, it is necessary that after the lapping, polishing is carried out to improve the surface roughness and, at the same time, to remove crystal strains and residue stresses brought about by the lapping process. However, with the previously described lapping method, the crystal strains and the residue stresses are so large that a relatively large machining allowance is required during the polishing process and this tends to constitute a cause of shape deterioration after the polishing process.
In other words, while an objective of the polishing process that is effected subsequent to the lapping process is to remove flaws and a residue stress layer resulting from the lapping process, to thereby provide a surface free from crystal flaws and residue stresses, the polishing process in which CMP (chemical machine polishing) is utilized involves a first problem in which because of difference in chemical action due to crystalline orientation, the shape precision such as a sphericity tends to degrade.
A second problem inherent in the polishing process and associated with the polished balls will now be discussed.
In the manufacture of the silicon wafer, polishing is generally performed as a final process so that the silicon wafer can be finished to have a surface having a surface roughness not greater than, for example, 5 angstrom and free from crystal strains. Where silicon balls are to be used as semiconductors, as discussed hereinbefore, the surface property comparable to the silicon wafer and the high shape precision are required.
In the case of the silicon balls, although the processing using the surface plates is most suited for the production of highly accurate balls, this method in which abrasive particles are used to achieve a mechanical grinding is ineffective to remove minute surface flaws and processing strains at a final stage and, therefore, the polishing is essential as a final process.
However, no polished balls of single crystal silicon that can be used as such semiconductor circuit substrates or the like have not yet been made available.
As a method of polishing the silicone balls, a method is known in which the workpieces are retained by a resinous retainer and are polished by a cloth of a kind generally used for polishing wafers. However, this method is susceptible to degradation of the shape, and the workpieces have to be supported in individually separated fashion, resulting in reduction in productivity.
The polishing is a method of grinding performed by the utilization of a chemical action without accompanying strains and residue stresses generated. However, where the workpieces to be processed is globular, processing of the workpieces while the latter are sandwiched between opposed flat polishing clothes tends to result in failure of a function of shape retention and, hence, the shape tends to considerably change. Also, the workpiece retainer is required to retain the workpieces and the workpieces are mounted one by one on the workpiece retained, resulting in reduction in productivity. These are a second problem.
A primary object of the present invention is to substantially solve the first problem discussed above to thereby provide lapped balls of single crystal silicon that can easily be manufactured so as to satisfy the requirements necessary for them to be used in semiconductor circuit substrate or the like, and also to thereby provide a method of easily making the lapped balls of single crystal silicon that have a required sphericity and a surface property.
Another important object of the present invention is to substantially solve the second problem discussed above to thereby provide polished balls of single crystal silicon of a high productivity that are highly accurate and free from strains and residue stresses in a processed surface and that can be used as semiconductor circuit substrates, solar cells or the like and also to thereby provide a method of making at a high productivity and highly accurately the polished balls of single crystal silicon that are free from strains and residue stresses in the processed surface.
In order to accomplish the foregoing objects, the present invention in one aspect provides a lapped ball of single crystal silicon having a sphericity of not greater than 0.08 xcexcm and also having a residue stress layer of not greater than 5 xcexcm in a depth from a processing surface thereof on one side in negative and positive directions.
The sphericity referred to above is preferably not greater than 0.05 xcexcm and more preferably not greater than 0.03 xcexcm, and the residue stress layer referred to above is preferably not greater than 2.5 xcexcm and more preferably not greater than 1 xcexcm.
The lapped ball of the present invention is, after having been finished by a subsequently performed post process such as, for example, polishing to produce a globular semiconductor circuit substrate, used as a semiconductor circuit element or the like. In the event that the lapped ball fails to satisfy the sphericity of not greater than 0.08 xcexcm during the lapping process, the extent to which the shape degrades after the polishing process would be considerable as compared with the case when the sphericity is good. Also, in the event that the residue stress layer does not satisfy the specific value of 5 xcexcm on one side, the polishing processing time and the polishing mount would increase and, as a result, reduction of the shape precision after the polishing would be considerably as compared with the case in which the residue stress layer satisfies the specific value of 5 xcexcm, The shape precision after the polishing process brings about a considerable influence on an IC manufacturing process that follows. By way of example, because of the bad shape, a beam used during exposure does not focus and, consequently, an integrated circuit will not be formed according to a design pattern, which eventually result in the integrated circuit failing to operate properly. Accordingly, the sphericity and the residue stress layer of the lapped ball, which would bring about influence on the shape precision after the polishing must be properly supervised and, therefore, the foregoing requirements have to be satisfied.
According to another aspect of the present invention, there is provided a method of making lapped balls of single crystal silicon by the use of a pair of lapping tables each prepared by hardening abrasive particles with a resinous bonding material and supported in face-to-face relation with each other, said method comprising a step of lapping workpieces of single crystal silicon between the lapping tables.
Where a surface plate made of abrasive particles hardened with the use of the resinous bonding material is used for each of the lapping tables, the processing with finely divided particles that has not hitherto been achieved with the mere resinous lapping tables can be achieved by fixing the abrasive particles with the use of the resinous bonding material, while securing advantages brought about by the resinous tables which do not result in agglutination with the workpieces. At the same time, since elastic deformation of the polishing tables is minimized, the shape precision can be increased. Also, the processing is possible with the very finely divided abrasive particles, accompanied by reduction in damage to the workpieces and, therefore, the crystal strains and the layer susceptible to the residue stress can be reduced.
In the practice of the present invention, at least one of the lapping tables supported in face-to-face relation with each other preferably has a workpiece rolling groove defined therein for rolling the workpieces of single crystal silicon.
The use of the lapping tables, at least one of which has the workpiece rolling grooves defined therein, is effective to secure the shape precision of the workpieces and the advantage brought about by the use of the lapping tables prepared from a surface plate containing the abrasive particles fixed with the use of the resinous bonding material can be exercised effectively.
According to a further aspect of the present invention, there is provided a polished ball of single crystal silicon having a sphericity of not greater than 0.5 xcexcm and a surface roughness of not greater than 0.5 nmRa and free from crystal strains and residue stresses in a region 3 xcexcm depthwise of a surface layer thereof.
The sphericity referred to above is preferably not greater than 0.3 xcexcm and more preferably not greater than 0.1 xcexcm, and the surface roughness referred to above is preferably not greater than 0.4 mmRa and more preferably not greater than 0.3 mmRa.
Preferably, the polished ball of single crystal silicon is the one obtained by processing the lapped ball of single crystal silicon of the kind described above in accordance with the present invention. The polished ball can be used as a semiconductor circuit substrate, a solar cell or the like as a component itself in the form as produced.
In a process of forming integrated circuits in such a globular element, crystal flaws such as crystal strains and residue stresses and the shape precision of the ball are extremely important.
Where the crystal flaws are present, the crystal flaws brings about a considerably adverse influence on the IC manufacturing process. By way of example, where a film of oxide is to be formed on a surface layer, formation of a film on a defect portion of a surface layer of silicon will become defective and/or an internal portion where crystal flaws are present will result in change in electric characteristic to such an extent as to cause the integrated circuit to operate improperly. Since in the case of the polished ball of single crystal silicon, the crystal flaws are all introduced from a processing surface (a spherical surface) and, therefore, absence of defects on the surface layer can provide an indication that there is no internal crystal flaws inside the ball.
The single crystal silicon has a spacing between the crystalline planes which is 0.54 nm (5.4 angstrom) and, if no flaw is present on the surface, s surface roughness smaller than the spacing between the crystalline planes of the single crystal ought to have been obtained. Accordingly, the polished surface free from surface crystal flaws satisfies the above described specific values (that is, the sphericity of not greater than 0.5 xcexcm and the surface roughness not greater than 0.5 nmRa).
If the sphericity is greater than the above described specific value, that is, 0.5 xcexcm, a considerably adverse influence will be brought about on the IC manufacturing process. By way of example, a beam used during exposure does not focus and, consequently, an integrated circuit will not be formed according to a design pattern, which eventually result in the integrated circuit failing to operate properly.
In view of the foregoing, the polished ball to be used in the semiconductor circuit substrate or the like must satisfy the foregoing requirements in connection with the crystal flaws, the sphericity and the surface roughness, and only when the foregoing requirements are satisfied, it can be used in the semiconductor circuit substrate.
According to a still further aspect of the present invention, there is provided a method of making polished balls of single crystal silicon with the use of a pair of polishing tables, one or both of said polishing tables comprising a polishing cloth and one or both of said polishing tables having a workpiece rolling grooves defined therein, said polishing tables being supported in face-to-face relation with each other, said method comprising polishing workpieces of single crystal silicon between the polishing tables.
According to this polishing method, since the polishing cloth is used, a high surface property comparable to the silicon wafer can be obtained. Also, since one or both of the polishing table are formed with the workpiece rolling groove, any possible degradation of the shape during the processing can be suppressed advantageously, and no workpiece retainer is necessary, resulting in an excellent productivity.
In the practice of the present invention, one of the polishing tables may comprise a polishing cloth and the other of the polishing tables comprise a resinous table.
Even when only one of the polishing table comprises the resinous table, the polished ball comparable to that obtained by the use of the polishing tables each comprising a polishing cloth can be manufactured.
Also, in the practice of the present invention, colloidal silica may be used as a processing fluid, and/or the polishing cloth may be made of foamed polyurethane.